Radiation sources for electromagnetic waves have been increasing in number with a continued advances in industrial technology and the general standard of living. The leakage of electromagnetic waves, therefore, has become a significant social problem since it exerts adverse influences on human bodies. Such leakage may also cause the incorrect behavior of integrated circuits in electronic equipments. In particular, electromagnetic waves radiated from electronic computers and various office equipment exert adverse influences on television sets and audio equipments.
Various methods have been used recently to shield such equipments against electromagnetic waves.
In general, since metals can absorb or reflect electromagnetic waves, they are effectively used as shielding materials for electromagnetic waves radiated from an electronic oven and various communication apparatuses. In addition, plastics with metals vapor-deposited, wire spray coated, sprayed or plated thereon can be used for the same purpose. Furthermore, materials prepared by adding relatively large amounts of additives, e.g., carbon powder and metal powder, to plastics can be used.
Such methods of using metals as such or of applying treatments, e.g., vapor deposition of metals, on plastics are disadvantageous in that the specific gravity is large, workability is poor, the procedure of treatment is not simple, and the treatment cost is high. Similarly, the method of introducing additives is disadvantageous in that when these additives are added in small amounts, the effect cannot be exhibited sufficiently. However, when they are added in large amounts, the mechanical strength and workability of the resulting molded article are seriously reduced although the effect can be exhibited.
Polyolefin compositions comprising metal powders and polyolefin or modified polyolefin are proposed, which have minimized reduction in mechanical strength to the extent that the reduction does not become a substantial obstacle for practical use. In addition, the compositions have good workability. See, for example, British Pat. Nos. 1,073,567, 1,168,701, 1,195,076, 1,246,829 and 1,226,181 and U.S. Pat. Nos. 3,468,753, 3,491,056 and 3,393,257. These polyolefin compositions have advantageous properties of electromagnetic interference. However, when they are used as a housing material for home electric appliances, e.g., a video camera, a television, and a hair drier, or as office equipments, e.g., a facsimile, a micro-computer, a printer and a word-processor, they are required to have flame resistance as well as properties of electromagnetic interference.
In general, in order to increase the effect of electromagnetic interference of synthetic resins or rubbers, it is necessary to incorporate a large amount of electrically conductive carbon black. It is, however, very difficult to provide flame resistance to compositions containing large amounts of carbon black. Even though additives such as halogen-containing organic compounds and antimony oxide are added to provide flame resistance, when the resulting composition is exposed to a flame in accordance with a flame resistance testing method ("UL-94 Method", Underwriter Laboratories, U.S.A.), glowing due to carbonization of the carbon black is vigorous. Furthermore, the flame remains alive for a long period of time, and if other flammable substances are brought into contact with the flame, they will easily catch fire. Even if considerable amounts of flame retarders are added this tendency remains and it is quite difficult to eliminate these defects. Moreover, the addition of such large amounts of flame retarders to compositions with carbon black incorporated thereinto causes a reduction in the properties of electromagnetic interference since it results in a reduction in the proportion of carbon black, even if flame resistance can be provided. Therefore, it is not possible to obtain a polyolefin composition with good electromagnetic interference properties which also has excellent flame resistance. Furthermore, the use of such large amounts of expensive flame retarders gives rise increased costs. In addition, a molded article prepared from the composition does not have mechanical characteristics which are suitable for practical use.